1. Field:
Disclosed are techniques for utilizing memory devices.
2. Information:
Non-volatile memories are a class of memory in which the memory cell or element does not lose its state after power supplied to the device is removed. The earliest computer memories, made with rings of ferrite that could be magnetized in two directions, were non-volatile, for example. As semiconductor technology evolved into higher levels of miniaturization, the ferrite devices were abandoned for the more commonly known volatile memories, such as DRAMs (Dynamic Random Access Memories) and SRAMs (Static-RAMs).
One type of non-volatile memory, electrically erasable programmable read-only memory (EEPROM) devices have large cell areas and may require a large voltage (e.g., from 12.0 to 21.0 volts) on a transistor gate to write or erase. Also, an erase or write time is typically of the order of tens of microseconds. One limiting factor with EEPROMs is the limited number of erase/write cycles to no more than slightly over 600,000—or of the order of 105-106. The semiconductor industry has eliminated a need of a pass-gate switch transistor between EEPROMs and non-volatile transistors by sectorizing a memory array in such a way that “pages” (e.g., sub-arrays) may be erased at a time in EEPROMs called flash memory devices. In flash memory devices, an ability to keep random access (erase/write single bits) was sacrificed for speed and higher bit density.
More recently, FeRAMs (Ferroelectric RAMs) have provided low power, relatively high write/read speed, and endurance for read/write cycles exceeding 10 billion times. Similarly, magnetic memories (MRAMs) have provided high write/read speed and endurance, but with a high cost premium and higher power consumption. Neither of these technologies reaches the density of flash memory devices, for example. As such, flash remains a non-volatile memory of choice. Nevertheless, it is generally recognized that flash memory technology may not scale easily below 65 nanometers (nm); thus, new non-volatile memory devices capable of being scaled to smaller sizes are actively being sought.
Technologies considered for the replacement of flash memory devices have included memories based on certain materials that exhibit a resistance change associated with a change of phase of the material (determined, at least in part, by a long range ordering of atoms in the crystalline structure). In one type of variable resistance memory called a phase change memory (PCM/PCRAM) devices, a change in resistance occurs as the memory element is melted briefly and then cooled to either a conductive crystalline state or a non-conductive amorphous state. Typical materials vary and may include GeSbTe, where Sb and Te can be exchanged with other elements of the same or similar properties on the Periodic Table. However, these resistance-based memories have not proved to be commercially useful because their transition between the conductive and the insulating state depends on a physical structure phenomenon (e.g., melting at up to 600 degrees C.) and returning to a solid state that cannot be sufficiently controlled for a useful memory in many applications.
Another variable resistance memory category includes materials that respond to an initial high “forming” voltage and current to activate a variable resistance function. These materials may include, for example, PrxCayMnzOϵ, with x, y, z and ϵ of varying stoichiometry; transition metal oxides, such as CuO, CoO, VOx, NiO, TiO2, Ta2O5; and some perovskites, such as Cr; SrTiO3. Several of these memory types exist and fall into the resistive RAMs (ReRAMs) or conductive bridge RAMS (CBRAM) classification, to distinguish them from the chalcogenide type memories. It is postulated that resistance switching in these RAMs is due, at least in part, to the formation of narrow conducting paths or filaments connecting the top and bottom conductive terminals by the electroforming process, though the presence of such conducting filaments are still a matter of controversy. Since operation of a ReRAM/CBRAM may be strongly temperature dependent, a resistive switching mechanism in a ReRAM/CBRAM may also be highly temperature dependent. Additionally, these systems may operate stochastically as the formation and movement of the filament is stochastic. Other types of ReRAM/CBRAM may also exhibit unstable qualities. Further, resistance switching in ReRAM/CBRAMs tends to fatigue over many memory cycles. That is, after a memory state is changed many times, a difference in resistance between a conducting state and an insulative state may change significantly. In a commercial memory device, such a change may take the memory out of specification and make it unusable.
Given an inherent difficulty in forming a thin film resistance switching material that is stable over time and temperature, a workable resistance switching memory remains a challenge. Furthermore, all resistance switching mechanisms developed up to now have been inherently unsuitable for memories, due to high currents, electroforming, no measurable memory read or write windows over a reasonable range of temperatures and voltages, and many other problems such as stochastic behavior. Thus, there remains a need in the art for a non-volatile memory that is deterministic has low power, high speed, high density and stability, and in particular, such a memory that is scalable to feature sizes well below 65 nanometers (nm).